The present invention relates to an organic electroluminescent device having an improved emission property that allows the device to emit color light having high color purity.
Organic electroluminescent devices (which may be referred to simply as xe2x80x9corganic EL devices,xe2x80x9d hereinafter) are spontaneous luminescence devices that operate taking advantage of the principle that fluorescent materials emit light due to recombination energy generated by recombination of the holes injected from an anode and the electrons injected from a cathode upon application of an electric field therebetween.
Since C. W. Tang of Eastman Kodak and his colleagues first reported about a low-voltage operated organic EL device using a double layered structure (100; 50 BC. W. Tang, S. A. VanSlyke, Applied Physics Letters, Vol. 51, 913, 1987), a significant amount of effort has been devoted to the study of organic EL devices that use organic materials.
Tang et al., used tris(8-hydroxyquinolinol aluminum) in a luminescent layer and a triphenyldiamine derivative in a hole transporting layer. The multi layered structure have several advantages: it offers a high efficiency at which holes are injected into the luminescent layer; it offers a high efficiency at which excitons are formed when electrons injected from the cathode are blocked to recombine; and the structure makes it possible to confine excitons generated in the luminescent layer. Among known device structures of organic EL devices are the double layer structure, which consists of a hole transporting (injection) layer and an electron transporting luminescent layer, and the triple layer structure, which consists of a hole transporting (injection) layer, a luminescent layer, and an electron transporting (injection) layer. To improve the efficiency at which the injected holes and electrons recombine in the laminated organic EL devices, numerous modifications have been made to the device structure, as well as to the manner in which each layer is formed.
As a hole transporting material for use in the hole transporting layer, derivatives of triphenylamine and aromatic diamine, such as 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine and N,N-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine, each known as a starburst molecule, have been disclosed in Japanese Patent Laid-Open Publications Nos. Hei 8-20771, Hei 8-40995, Hei 8-40997, Hei 8-53397, and Hei 8-87122.
On the other hand, derivatives of oxadiazole and triazole are known as electron transporting materials for the electron transporting layer.
As a luminescent material for use in the luminescent layer, different luminescent materials are known, including chelate complexes, such as tris(8-quinolinolato)aluminum complex, coumarin derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, and oxadiazole derivatives. It is described in Japanese Patent Laid-Open Publication Nos. Hei 8-239655, Hei 7-138561 and Hei 3-200889 that visible light, ranging from blue to red, can be obtained using these luminescent materials, and their application to the development of color display devices is expected.
Recently, high luminance organic EL devices that offer a high emission efficiency have been developed. However, the light that these highly efficient, high luminance organic EL devices emit may contain light emanating from the layers other than the luminescent layer, including the electron transporting layer. As a result, the color purity of the emitted light is often decreased.
This problem is particularly significant in blue luminescent devices. For example, although several luminescent materials are known to emit pure blue light, the light emanating from these blue luminescent materials may overlap the light emitted by the electron transporting layer when the materials are used in the luminescent layers of organic EL devices. Consequently, desired blue luminescence is often not obtained in these organic EL devices.
The present invention has been devised in view of the above-described situation and one of its objectives is thus to provide a highly efficient high luminance organic electroluminescent device that can emit light with a high color purity.
In an effort to find a solution to the above-described problems, the present inventors have made a finding that the amount of light emission from layers other than the luminescent zone luminescent layer) can be significantly decreased by disposing, between an anode and a cathode, a thin layer of organic film that contains a particular compound that includes a condensed polycyclic hydrocarbon to serve as a linking group. In this manner, highly efficient, high luminance organic electroluminescent devices capable of emitting light with high color purity can be achieved. The compound that includes a condensed polycyclic hydrocarbon linking group is a good electron transporter and, thus, a thin layer of organic film containing this compound can be used in organic electroluminescent devices in place of the conventional electron transporting layer to function as an electron transporting layer. This finding ultimately led the present inventors to complete the present invention.
The present inventors have also found that, by disposing between an anode and a cathode a thin organic film layer that contains a certain polyphenylene compound with cyclohexylidenemethine groups, the amount of light emission from the layers other than the luminescent zone (luminescent layer) can be significantly decreased and emission characteristics of electroluminescent device can be improved and, therefore, highly efficient, high luminance organic electroluminescent devices capable of emitting light with high color purity can be achieved.
It is believed that introduction of cyclohexylidenemethine groups as a substituent of the polyphenylene compound raises the electron injection level of the polyphenylene compound and for this reason, the deposition of the organic thin film layer between the anode and the cathode can improve the electron injection properties and device performance (emission characteristics) of organic electroluminescent devices.
The present inventors have also found that certain polyphenylene compounds that have cyclohexylidenemethine groups exhibit a high electron transporting property and, therefore, an organic thin film layer containing this compound may be used in place of the conventional electron transporting layer to function as an electron transporting layer. In this manner, the present inventors have invented an organic electroluminescent device having the following construction.
Accordingly, the organic electroluminescent device of the present invention is characterized in that it includes at least one organic thin film layer disposed between an anode and a cathode and include a luminescent zone that includes at least one luminescent layer. At least one of the organic thin film layers contains a compound that is represented by one of the following general formulae [I], [II], and [III] and may include a cyclohexylidenemethine group:
A1-X1-(A2)axe2x80x83xe2x80x83[I]
XN-(Y)nxe2x80x83xe2x80x83[II]
Y1-Y2.xe2x80x83xe2x80x83[III]
In the general formula [I], a is 0 or 1, X1 is a monovalent or divalent linking group, which may or may not be a condensed polycyclic hydrocarbon, and A1 and A2 are each independently a substituted or unsubstituted polyphenylene unit represented by the general formula [IV]: 
wherein b is an integer from 0 to 18.
In the general formula [II], XN is a linking group having a valency of 2 to 4, and (Y)n represents n groups (wherein n is an integer from 2 to 4), which are each independently represented by the general formula [I]. In the general formula [III], Y1 and Y2 are each independently a group represented by the general formula [I].
Preferably, X1 in the general formula [I] is a divalent linking group provided in the form of any of naphthalene, fluorene or phenanthroline from which two hydrogen atoms are eliminated.
Preferably, XN in the general formula [II] is a linking group having a valency of 2 to 4 and provided in the form of any of acyclic hydrocarbon, monocyclic hydrocarbon, condensed polycyclic hydrocarbon, crosslinked cyclic hydrocarbon, Spiro hydrocarbon, or ring assembly hydrocarbon with 2 to 4 hydrogen atoms eliminated therefrom. Of these, preferred are divalent linking groups provided in the form of one of fluorene, benzene, naphthalene, phenanthrene, adamantine, or cyclohexane from which two hydrogen atoms are eliminated.
Preferably, an organic thin film layer containing a compound represented by one of the general formulae [I], [II], and [III] is interposed between the luminescent zone and the cathode in the organic electroluminescent device of the present invention. The present inventor has found that such construction is effective in preventing light emission from the layers other than the luminescent zone and thereby further improving the color purity of the emitted light.
The present inventor has also discovered that, in conventional organic electroluminescent devices, light emission from the electron transporting layer is most significant among all the other layers except for the luminescent zone and that, by disposing an organic thin film layer containing a compound represented by one of the general formulae [I], [II], and [III] either between the luminescent zone and the electron transporting layer or between the electron transporting layer and the cathode, the amount of light emission from the electron transporting layer can be decreased and, hence, the color purity of the light emitted by the device can be effectively improved.
The present inventor has further found that the present invention is particularly effective when the luminescent zone is arranged adjacent to the anode and, in such a case, the color purity of the emitted light can be further improved.
The present inventor has also found that, in the organic luminescent devices of the present invention, the organic thin film layer containing the particular compound represented by one of the general formulae [I], [II], and [III] (i.e., the particular compound including a condensed polycyclic hydrocarbon linking group, or the particular polyphenylene compound including a cyclohexylidenemethine group) can be readily doped with metal and that, by doping such an organic thin film layer containing the compound with metal, the amount of light emission from the layers other than the luminescent zone can be further reduced through the interaction between the compound and the metal and, consequently, the electron transporting property of the organic thin film layer containing the compound can be improved, as can the emission property of the organic electroluminescent device (e.g., luminance and emission efficiency).
Preferably, the metal to dope the organic thin film layer containing the compound represented by one of the general formulae [I], [II], and [III] is one or more selected from alkali metals such as Li, Na, K, Rb, Cs, and Fr, alkali earth metals such as Mg, Ca, Sr, Ba, and Ra, Al, Ga, In, and Tl.